全文获取类型
收费全文 | 4382篇 |
免费 | 169篇 |
国内免费 | 11篇 |
专业分类
测绘学 | 191篇 |
大气科学 | 652篇 |
地球物理 | 1011篇 |
地质学 | 1815篇 |
海洋学 | 178篇 |
天文学 | 545篇 |
综合类 | 18篇 |
自然地理 | 152篇 |
出版年
2021年 | 65篇 |
2020年 | 63篇 |
2019年 | 44篇 |
2018年 | 143篇 |
2017年 | 164篇 |
2016年 | 236篇 |
2015年 | 136篇 |
2014年 | 198篇 |
2013年 | 255篇 |
2012年 | 109篇 |
2011年 | 174篇 |
2010年 | 189篇 |
2009年 | 226篇 |
2008年 | 158篇 |
2007年 | 125篇 |
2006年 | 116篇 |
2005年 | 95篇 |
2004年 | 63篇 |
2003年 | 70篇 |
2002年 | 102篇 |
2001年 | 86篇 |
2000年 | 66篇 |
1999年 | 72篇 |
1998年 | 71篇 |
1997年 | 63篇 |
1996年 | 53篇 |
1995年 | 69篇 |
1994年 | 78篇 |
1993年 | 31篇 |
1992年 | 32篇 |
1991年 | 33篇 |
1990年 | 41篇 |
1989年 | 35篇 |
1988年 | 35篇 |
1987年 | 33篇 |
1985年 | 44篇 |
1984年 | 46篇 |
1983年 | 54篇 |
1982年 | 42篇 |
1981年 | 44篇 |
1980年 | 37篇 |
1979年 | 28篇 |
1978年 | 55篇 |
1977年 | 37篇 |
1976年 | 32篇 |
1975年 | 43篇 |
1974年 | 52篇 |
1973年 | 47篇 |
1970年 | 26篇 |
1969年 | 26篇 |
排序方式: 共有4562条查询结果,搜索用时 31 毫秒
1.
Himmerfjärd is a Swedish estuary bordering on the Baltic. The estuary lacks astronomical tides and its circulation is driven by winds and freshwater runoff. Because of a tertiary sewage treatment plant located at its inner end, the estuary is becoming increasingly eutrophic. A field study was carried out for a 78-day period in late summer and early fall of 1977 to determine rates of nutrient transport and to construct nutrient budgets. Since physical parameters (current velocity, temperature, salinity, winds and water level changes) were measured more frequently than nutrients (phosphate, nitrate, nitrite and ammonium) it was necessary to develop a suitable method to calculate nutrient flux time series and net nutrient fluxes. Over the study period, Himmerfjärd imported phosphorus and exported nitrogen. Direction of nutrient fluxes and changes in flux direction were consistent with the structure of the baroclinic currents. 相似文献
2.
Veined lithologies are formed by fracturing and sealing processes, with the veins representing former fluid conduits through the rock. Although detailed fieldwork and numerical simulations have provided a better understanding of vein growth, few studies have attempted to seal fractures and generate veins experimentally. In this pilot study, we subjected fractured quartzite to temperature gradients of 45–125 °C under hydrothermal conditions in a static fluid, with the aim of precipitating secondary quartz in the cooler portions of the fracture. Results show that secondary quartz precipitates due to the imposed temperature gradient, causing the initial fracture to seal locally. Although no systematic sealing pattern was observed along the fracture, samples subjected to higher temperatures exhibit a smaller fracture width and appear to have reacted more extensively. Electron microprobe mapping visualizes the spatial distribution of secondary quartz, which contains elevated concentrations of aluminium. 相似文献
3.
4.
Gösta Gahm 《Astrophysics and Space Science》2006,304(1-4):149-152
The number of confirmed and suspected close T Tauri binaries (period days) is increasing. We discuss some systems with enhanced emission line activity and periodic line profile changes. Non-axisymmetric flows of plasma in the region between the circumbinary disk and the stars can be generated through the influence of the secondary component. Such enhanced activity is found around binaries with eccentric as well as circular orbits. We discuss our observations of the T Tauri stars RW Aurigae A and RU Lupi, which may host very close brown dwarf companions. Model simulations indicate that non-axisymmetric flows are generated around close binaries with circumbinary disks, also in systems with circular orbits. 相似文献
5.
Joachim Köppen 《Astrophysics and Space Science》2003,284(2):837-840
Representative results from a comparison of the chemical evolution of spherical collapse models without and with a intercloud
medium are presented. The hot metal-rich gas distributes quickly the metals produced in supernovae throughout the galaxy,
thus leading to a more homogeneous chemical evolution and flatter metallicity gradients in the gas and the stars. The stellar
population is somewhat less concentrated towards the centre. The strong outflow results in a substantial loss of metals from
the galaxy to its surroundings, and a lower effective yield in the galaxy.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
6.
We present warm dark matter (WDM) as a possible solution to the missing satellites and angular momentum problem in galaxy
formation and introduce improved initial conditions for numerical simulations of WDM models, which avoid the formation of
unphysical haloes found in earlier simulations. There is a hint, that because of that the mass function of satellite haloes
has been overestimated so far, pointing to higher values for the WDM particle mass.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
7.
Arnold Gucsik Christian Koeberl Franz Brandsttter Eugen Libowitzky Wolf Uwe Reimold 《Meteoritics & planetary science》2003,38(8):1187-1197
Abstract— We studied unshocked and experimentally (at 12, 25, and 28 GPa, with 25, 100, 450, and 750°C pre‐shock temperatures) shock‐metamorphosed Hospital Hill quartzite from South Africa using cathodoluminescence (CL) images and spectroscopy and Raman spectroscopy to document systematic pressure or temperature‐related effects that could be used in shock barometry. In general, CL images of all samples show CL‐bright luminescent patchy areas and bands in otherwise nonluminescent quartz, as well as CL‐dark irregular fractures. Fluid inclusions appear dominant in CL images of the 25 GPa sample shocked at 750°C and of the 28 GPa sample shocked at 450°C. Only the optical image of our 28 GPa sample shocked at 25°C exhibits distinct planar deformation features (PDFs). Cathodoluminescence spectra of unshocked and experimentally shocked samples show broad bands in the near‐ultraviolet range and the visible light range at all shock stages, indicating the presence of defect centers on, e.g., SiO4 groups. No systematic change in the appearance of the CL images was obvious, but the CL spectra do show changes between the shock stages. The Raman spectra are characteristic for quartz in the unshocked and 12 GPa samples. In the 25 and 28 GPa samples, broad bands indicate the presence of glassy SiO2, while high‐pressure polymorphs are not detected. Apparently, some of the CL and Raman spectral properties can be used in shock barometry. 相似文献
8.
Judit Györgyey 《Celestial Mechanics and Dynamical Astronomy》1985,36(3):281-285
The non-linear stability of motions around L5 in the elliptic restricted problem of the three bodies is investigated numerically with emphasis on the effect of the orbital eccentricity of the primaries on the shape of the established stability regions. It is shown that with increasing eccentricity, the width of these regions is decreasing. 相似文献
9.
The lifetimes, characteristics of the shapes as well as lengths and perimeters of artificial solar granules (Nordlund, 1982, 1984a) are compared with data from the literature and parameters determined from two different sets of observed granules. No significant differences of the parameters for these sets of granules are detectable.Mitteilungen aus dem Kiepenheuer-Institut Nr. 251. 相似文献
10.
Mussels, Mytilus edulis L., were exposed to elevated concentrations of copper or cadmium in the laboratory, then placed in cages in the sea (salinity 7‰). One year later maximum lengths of the mussels were measured and shells screened for deformities. Growth was 0·6 cm year?1 in the control cages and retarded in cages of exposed mussels. A total of 63% of cadmium-exposed and 46% of copper-exposed mussels had shell deformities. In the control cages 26% were deformed while in a natural population only 3% were deformed. The proportion of deformities to growth was inversely related. Low concentrations showed their injuriousness in this long-term test, probably due to the closing of the valves during exposure to high concentrations. 相似文献